Aqueous polymer dispersions produced by free-radically initiated aqueous emulsion polymerization, also referred to hereinafter as emulsion polymers, find various uses for production of coating compositions and adhesives, and as a binder for fibers and textiles.
When aqueous polymer dispersions are used for production of aqueous coating compositions such as emulsion paints and varnishes, it is often the case that properties achievable only by contrary measures are desirable. On the one hand, the polymer dispersions are to assure good film formation at low temperatures, even in the case of low proportions of filming auxiliaries, which typically entails a low glass transition temperature of the polymer. At the same time, a high hardness and scratch resistance of the dry film is required, which is typically assured only by means of comparatively hard polymers having a correspondingly high glass transition temperature, which form films only with difficulty, if at all, without filming auxiliaries.
There have been various proposals of multistage polymer latices, i.e. polymer dispersions of polymer particles composed of several polymer phases having different glass transition temperatures, for the production of wood coatings—see, for example, EP 184091, EP 623659 A1, EP 710680 A1 or EP 795568 A2. The multistage polymer latices are generally produced by a multistage emulsion polymerization in which the monomers which form the respective polymer phase are polymerized sequentially, i.e. in successive polymerization stages. Hereinafter, multistage polymer latices are therefore also referred to as multistage emulsion polymers. The multistage emulsion polymers described in the documents cited are suitable for pigmented wood paints and varnishes and feature good blocking resistance and in some cases also very low water sensitivity.
Even though it is possible to use the aforementioned multistage emulsion polymers as binders in coating systems for wood and wood-like substrates, certain disadvantages nevertheless arise, especially when they are formulated in pigmented or tinted wood paints. Thus, the aforementioned multistage emulsion polymers are incompatible with various pigment types frequently used in such color formulations, which is manifested in the formation of coagulate and filter residues in the course of production of the formulation, and by inadequate hue stability on application of the coating to the substrate. Furthermore, it is often possible only with difficulty to adjust the viscosity characteristics of the formulation, which are important for proper application of the paint to the substrate.
WO 2008/152017 describes the production of aqueous multistage emulsion polymers, in which an emulsion polymer is first prepared, formed exclusively from monomers having limited water solubility, and then two monomer compositions are polymerized onto it successively. The process allows control of the particle size of the polymer and gives emulsion polymers having good mechanical properties such as blocking resistance, which are manifested in pigment-containing paint formulations by good color brightness and color depth. However, hue stability is inadequate in some cases. Moreover, on application of the paint, leveling is disrupted, which leads to only moderate gloss of the dried paint film, which is problematic especially in the case of clearcoats.
WO 2009/095569 describes multistage emulsion polymers which are prepared by free-radical emulsion polymerization in the presence of phosphate emulsifiers. However, hue stability is likewise only inadequate, and leveling is likewise disrupted on application of the paint.